1. Field of the Invention
The present invention relates to a process and an apparatus for producing a substrate having a coating resin layer which excels in weather resistance and durability and which also excels in productivity.
2. Related Background Art
In the case where a substrate such as a plastic plate, a steel plate or a galvanized steel plate is used outdoors, there is often adopted such a manner that prior to using said substrate decorative coating is performed so that the substrate has improved weather resistance and improved durability. In this case, for instance, when the substrate is a steel plate or a galvanized steel plate, it is important that a coating material which is satisfactory in terms of the weather resistance and the durability is used for the decorative coating and the interface between the coat layer and the steel plate or the galvanized steel plate is sufficiently covered and protected. As specific examples of such substrate which is used outdoors, there can be mentioned a sign board in which a metalic plate is used, a roof material in which a metallic plate is used, and a solar cell in which a photovoltaic element which is provided on a metallic substrate is used.
Now, in the case of a solar cell which is installed outdoors, it generally has a structure in that a photovoltaic element is resin-sealed between a surface member and a back face member. FIG. 4 is a schematic cross-sectional view illustrating an example of the structure of such solar cell. A solar cell 400 having such structure as shown in FIG. 4 may be produced, for instance, in a manner in that a surface member 402, a surface side sealing member 403, a photovoltaic element 401, a back face side sealing member 404 and a back face member 405 are laminated in this order from the outermost surface side to obtain a stacked body and the stacked body is subjected to a thermocompression bonding treatment to obtain a solar cell 400.
Japanese Patent Publication No. 4(1992)-65556 and Japanese Registered Utility Model Publication No. 3017231 (issued Oct. 24, 1995) disclose a process for producing such solar call by using a double-vacuum type lamination apparatus. Besides, Japanese Laid-open Patent Publication No 9(1997)-36405 discloses a process for producing such solar cell by using a single-vacuum type lamination apparatus. However, these solar cell production processes using such lamination apparatus have disadvantages such that a plurality of lamination materials are necessary to be used and a large investment in plant and equipment is required, and therefore, the resulting solar cell unavoidably becomes costly, and this results in a raise in the power generation cost of the solar cell.
By the way, in recent years, aiming at reducing the power generation cost of a solar cell there have been made attempts in that a facility where a plurality of solar cells specialized solely with respect to power generation function are installed under controlled environment is used as a small-scale power generation station. The term “controlled environment” means a state in that the solar cells installed are supervised while being enclosed with a fence or a wall and locked so as to prohibit anybody from entering therein except for a specified handling person.
For the solar cells which are installed in such a controlled environment, the safety design adopted for ordinary solar cells for various use purposes in order to endure any accidental occurrence may be partly omitted, and this makes it possible to diminish the power generation cost. To be more specific, for ordinary solar cells which are installed outdoors for the power generation purpose, they are usually designed to have a covering material constitution such that the insulating property can be sufficiently maintained also against surface damages occurred when a person inexperience in the handling, for instance, accidentally scratches their light receiving surfaces. For the solar cells which are installed under the controlled environment, such situation is not necessary to be taken into consideration and therefore, their covering materials can be greatly diminished.
As an example of the solar cell having such covering structure as above described which is installed under the controlled environment, there can be considered a configuration in that a thin film resin layer having a minimum covering constitution which prevents the photovoltaic element of the solar cell from being directly contacted with moisture such as rain is provided in the power generation region of the photovoltaic element.
In order to improve the weather resistance and durability of the photovoltaic element having such configuration, it is important that not only the surface of the photovoltaic element but also the interfaces between exposed portions of the transparent electrode layer, photoelectric conversion layer, back reflecting layer and substrate at the edge portions of the photovoltaic element are resin-sealed.
As a simple method capable of resin-sealing such exposed portions at the edge portions of the photovoltaic element, there can be mentioned a resin-sealing method by way of coating with the use of a resin liquid and a resin powder, specifically, for instance, as described in Japanese Laid-open Patent publication No. 6(1994)-338627 which discloses a resin-sealing method by way of curtain coating and in Japanese Laid-open Patent publication No. 6(1994)-151936 which discloses a resin-sealing method using a slot orifice coater.
These resin-sealing methods are such that a photovoltaic element arranged on a conveyor belt is carried while passing through a resin material spouted in a curtain-like state to form a resin layer on the photovoltaic element. According to this technique, the resin material is uniformly deposited also on a portion of the conveyor belt which surrounds the photovoltaic element and because of this, the edge portions of the photovoltaic element can be resin-sealed.
However, any of the resin-sealing methods has such disadvantages as will be described in the following. That is, upon the resin-sealing treatment, the conveyor belt is stained by the resin material spouted and therefore, when the conveyor belt is continuously used, it is required to perform a cleaning step in order to clean the conveyor belt, independently from the coating step. This makes the apparatus complicated, and an extra investment therefor is necessitated.
Separately, Japanese Laid-open Patent publication No. 6(1994)-85299 discloses a sealing technique to seal a photovoltaic element by way of electrostatic spray painting. The sealing technique disclosed in this document is that using an electrostatic powder coating apparatus of either the corona electrical charging system or the frictional electrification system, charged resin powder is electrostaticaly deposited uniformly on the entire surface of a photovoltaic element carried by a conveyor belt or the like, the resin powder deposited on the photovoltaic element is heated and fused, followed by being cured, whereby sealing the photovoltaic element.
According to this sealing technique, the entire surface of the photovoltaic element including the end portion thereof which is electrically grounded can be resin-sealed. In addition, by charging the conveying means such as the conveyor belt to have a polarity which is the same as that of the resin powder, it is possible to prevent the conveying means from being stained with the resin powder. However, in this case, in order that the photovoltaic element is electrically grounded, a portion of the conveyor belt which is in contact with the photovoltaic element is necessary to be also electrically grounded. Because of this, said portion of the conveyor belt which is in contact with the photovoltaic element is also deposited with the charged resin powder, where the conveyor belt is unavoidably stained with the resin powder. When the resin sealing of the photovoltaic element is continuously performed awhile disregarding the incidence in that the conveyor belt is stained, the yield of a product is disadvantageously decreased.